The image on the television screen consists of pixels, arranged horizontally in rows, generally offset vertically by one pixel position from one another. Each pixel is assigned three values, which indicate the respective intensities of the red, green, and blue components of the pixel. A video image is generated by sequentially displaying the rows of pixels as horizontal lines of the image.
Existing analog broadcast standards such as NTSC, PAL and SECAM use two video fields to generate a single video frame. Each field includes one-half of the horizontal lines that make up the image frame. One field includes all of the odd numbered lines of the frame and the other field includes all of the even numbered lines. Interlaced images exhibit distorting artifacts such as vertical line flicker or loss of vertical detail that degrade the resulting frame image. One way to eliminate these artifacts is to convert the interlace-scanned fields into progressive-scan frames. In a progressive-scan frame, both the odd and even image lines are displayed sequentially as a single image.
Interlace-scan to progressive-scan conversion systems are gaining importance as more television viewers purchase high-definition television monitors that can display progressive-scan signals. Both broadcast facilities and consumers may want to have interlace-scan to progressive-scan conversion capability in order to avoid the distorting artifacts of interlace-scan images.
One way to generate a progressive-scan frame from an interlace-scan field is to interpolate interstitial lines in each field. Thus, the lines of the odd field are used to interpolate even-numbered lines and the lines of the even field are used to interpolate odd-numbered lines.
Each pixel of the interpolated line (or the “interpolated pixel”) is calculated based on the values of proximate pixels in adjacent interlace-scan lines. The simplest method of generating the interpolated pixel is simply duplicating the pixel from the corresponding position in the previously received scan line. For pixels which lie on a diagonal edge, this could result in “jaggies” (a line which appears to be jagged or stair-stepped, rather than smooth). For pixels which are not on an edge, such duplication could result in pixels that do not correspond to the image being displayed, resulting in a poor display to the viewer. This method also reduces the vertical resolution of the image compared to an interlace-scan image and may result in areas of the image flickering at a 30 Hz rate.
Another simple method is to set the value of the interpolated pixel as the average of two vertically adjacent pixels. However, for a pixel on the edge of two visually distinct regions, such an averaging could result in a pixel that matches neither adjacent pixel. For example, the value generated for an interpolated pixel between a blue pixel and green pixel may be cyan, which would not result in the image desired to be presented to the viewer.
FIG. 1 shows an image on a television image 100, which includes two visually distinct regions 102 and 104. The border 103 between the two visually distinct regions is referred to herein as an edge. An image on a television screen may consist of more than two visually distinct regions, and any one or more visually distinct regions may not be entirely contained within the television screen, as is illustrated.
Visually distinct regions are defined by the edge 103 between them, in contrast to a more gradual change, such as a shadow (which may have gradations of gray and black) or light on a wall (which may have gradations of color). In generating an interpolated pixel 108 which is to be on an edge, it is desirable to consider the visual smoothness of the edge. If the value of the pixel 108 on the interpolated line 106 were based solely on the pixels proximate in the received scan lines 105 and 107, the calculated value may be a blend of the values of the two visually distinct regions, rather than a distinctive edge separating the two regions. The result could be an edge without sufficient clarity to distinguish between the two regions, a line that is not visually smooth, or a pixel that has the correct value for an edge but is displayed at the wrong pixel location.